What did he know, and when did he know it?
In this prediction, Semmelweis did not know what the outcome would be. No one knew. In fact, the physicians were offended and outraged that their sacred profession could have been implicated.
Thus the element of surprise in a scientific result is essential, because the risk in the prediction is unavoidable. Before the process, Semmelweis did not know the answer. After the process, we all can know. This suggests that our sought-for certainty is a myth, at least in a scientific procedure.
Today, of course, hygiene in a hospital is routine, now that the phenomenon of infection is understood. But infection was not understood when Semelweis made his pioneering investigation. Now we can afford the luxury of confidence in the work of Semmelweis, even if we cannot afford a myth of certainty.